The present invention relates to a preparation method of a thermally developable photothermographic material and in particular to a photothermographic material exhibiting superior dimensional stability even after being thermally developed.
There have been made a number of studies of the thermally processing process in which development is carried out by heating to form a black-and-white image or a color image. There are also known so-called transfer type photothermographic materials in which images obtained by thermal development are transferred to an image receiving layer from the photothermographic material. In photographic materials employing a thermal development system, development is usually carried out at a temperature of 80 to 150xc2x0 C. and the dimensional change of the photographic material after being thermally developed is rather pronounced, compared to conventional wet-processed photographic materials, which produce problems in practical use.
Reduction of such marked dimensional change can be classified into two main methods. One of them is to enhance heat resistance of the photographic material and the other one is development of an image forming method to reduce a dimensional change during thermal processing.
Methods for enhancing heat resistance of photothermographic materials include, for example, a technique described in JP-A No. 10-10676 and 10-10677, in which a support is subjected to a thermal treatment while being transported at a high temperature of 80 to 200xc2x0 C. and a low tensile force of 0.04 to 6 kg/cm2 to lessen thermal shrinkage of the support, thereby reducing the dimensional change thereof. However, the thermal treatment under such a low tensile strength produced the thermal shrinkage of the support which is locally different, leading to deterioration of flatness of the support and fine abrasion marks caused by friction from the transport rollers, leading to lowered quality of the photothermographic material.
With regard to the image forming method of photothermographic materials, a number of techniques prescribing temperature stability at the time of thermal development were disclosed, for example, as described in JP-A 9-292695, but there is not disclosed an image forming method reducing the dimensional change.
Accordingly, it is an object of the present invention to provide an image forming method of a photothermographic material exhibiting a reduced dimensional change after being thermally developed.
As a result of the inventors"" study to solve the problems described above, it was found that when a photographic support was subjected to a thermal treatment under tension, varying the tensile force and preferably gradually reducing the tensile force resulted in reduction of the residual internal stress which was produced at the film-making stage of the support, for example, at the stage of stretching, leading to the support exhibiting lessened dimensional change and superior quality as exemplified by a flat surface of flatness.
Further, to prepare photothermographic materials, a solution containing an organic silver salt, photosensitive silver halide and a reducing agent is coated on a support and dried at a temperature of 40 to 80xc2x0 C., while transporting the support under tension. In this case, it was found that controlling the transport tension resulted in reduced dimensional change. It was further found that in thermal processing of the photothermographic materials, adjusting physical properties of the photothermographic material, the pressure on the transporting medium and the matting degree of the transporting medium resulted in lessening of the dimensional change.
The object of the present invention can be accomplished by the following constitution:
a method for preparing a photothermographic material comprising on a support an organic silver salt, photosensitive silver halide and a reducing agent, the method comprising:
(i) coating a coating solution containing the organic silver salt, the photosensitive silver halide and the reducing agent on a support to form a coated material, while the support is in tension, and
(ii) subjecting the coated material to a thermal treatment at a temperature of 40 to 120xc2x0 C.,
wherein in step (ii), a tension of 0.01 to 30 kg/m2 is applied to the support.